Aye Up Snowdon Bushwhacker,
1. You are welcome for the info. Here is some more.
2. Bivvy bags (mil 'breathable' versions) are intended not only to keep you dry and reasonably condensation free but also to not create the counter-tactical straight edges/lines that occur if you don't set a basher (tarp) up correctly - that is why mil bivvies usually either have no hoops or some (usually 1 or 2) at the head end. This then creates only a
diagonal line from head to foot - less eye catching = more tactical.
3. That method however causes the bag to 'drape' over the body/sleeping bag potentially creating heat escape through conduction. (I initially found that especially in heavy downpours and on several occasions after hard rain and then a frost!). A full length basher set up over the bivvy does lessen those issues
and provide some sheltered 'sit-up' room (depending on how you set it up).
4. Furthermore, if you occupy the bivvy in boots or are particularly restless, I found that I sometimes tended to get tangled in the material and roll the bag off the ground insulation mat. (On that subject I have long turned to using a ground
sheet beneath the bivvy and a ground
insulation mat inside it).
Introducing a hoop at the foot-end and a ridge-line with a central loop raises the bag slightly, keeps feet/boots clear and helps keep the bag up off you/your sleeping bag and doesn't interfere too much with the (tactical) diagonal line.
5. In the case of an obs bivvy it is likely that you are going to add external cam which you don't want moving about each time that you do inside the bag, so the foot-en hoop and the ridge-line help to minimise that.
6. The dutch single hooped bivvy was originally intended simply as a low profile, rapidly set up sleeping shelter - (limited use for obs) but I found that when using it as a bush-craft sleeping shelter, the inability to see out from the head end was (psychologically) uncomfortable so my mods on it as an obs shelter also helped for bush-craft use.
